I Have 2 tables WIthdrawMaterial and DepositMaterial,Both In WithdrawMaterial contain quantity column, and DepositMaterial contain totalDeposit ,I want to perform subtraction between quantity and totalDeposit column using query and store the result in pending column of TransactionMaterial table . 

This is my code,  what I tried to solve this issue.

package com.nilmani.workload.repository

import com.nilmani.workload.entity.TransactionMaterial
import org.springframework.data.jpa.repository.JpaRepository
import org.springframework.data.jpa.repository.Modifying
import org.springframework.data.jpa.repository.Query
import org.springframework.transaction.annotation.Transactional
import org.springframework.web.bind.annotation.RequestParam

interface TransactionRepository :JpaRepository<TransactionMaterial,Long> {

   @Query("SELECT  WithdrawMaterial .quantity,DepositMaterial .totalDeposit,WithdrawMaterial .quantity-DepositMaterial .totalDeposit  FROM WithdrawMaterial  INNER JOIN DepositMaterial ON WithdrawMaterial .id = DepositMaterial .withDrawId")
    fun pendingMaterial(@RequestParam("withDrawId") withDrawId:Long,@RequestParam("depositId")depositId:Long):Long

}

But this query Not working properly ,It throws null pointer exception while execute the application 

My entity class WithdrawMaterial Entity class

package com.nilmani.workload.entity

import com.nilmani.workload.enum.Material
import java.time.LocalDateTime
import javax.persistence.Entity
import javax.persistence.GeneratedValue
import javax.persistence.GenerationType
import javax.persistence.Id

@Entity
data class WithdrawMaterial(
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    val id:Long=-1,
    val user_id:Long=-1,
    val material:Int = Material.NONE.type,
    val quantity:Long=-1,
    val time:LocalDateTime= LocalDateTime.now(),
)

DepositMaterial Entity Class

package com.nilmani.workload.entity

import com.nilmani.workload.enum.Material
import java.time.LocalDateTime
import javax.persistence.Entity
import javax.persistence.GeneratedValue
import javax.persistence.GenerationType
import javax.persistence.Id

@Entity
data class DepositMaterial (
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    val id:Long=-1,
    val withDrawId:Long=-1,
    val totalDeposit:Long=-1,
    val material:Int= Material.NONE.type,
    val depositTime:LocalDateTime = LocalDateTime.now(),
    )

controllerClass

@PostMapping("/updateTransaction")
 fun updateWithdrawMaterial(@ModelAttribute request: ReqUpdateWithdrawMaterial):ResponseEntity<*>{
     val existingUser = employeeRepository.findById(request.userId)
     if (existingUser.isPresent){
         val existWork = withdrawMaterialRepository.findById(request.withDrawId)
         return if (existWork.isPresent){
             val pendingNearUser = TransactionMaterial(
                 userId = request.userId,
                 withDrawId = request.withDrawId,
                 material = request.material,
                 depositId = request.depositId,
                 pending = transactionRepository.pendingMaterial(request.withDrawId,request.depositId)
             )
             val saveMaterial = transactionRepository.save(pendingNearUser)
             return ResponseEntity(saveMaterial,HttpStatus.OK)
         }else{
             ResponseEntity(ResMessage("you need to learn then we provide work you"),HttpStatus.BAD_REQUEST)
         }
     }else{
         return ResponseEntity(ResMessage("You are not a registered User ! Register First"),HttpStatus.NOT_ACCEPTABLE)
     }
 }

What is the reason for this type of issue,I cannot find where I got null value ,I provide default value to all the variables ,While using query I got nullPointer Exception ,

Caused by: java.lang.NullPointerException: null
    at java.base/java.lang.String$CaseInsensitiveComparator.compare(String.java:1244) ~[na:na]
    at java.base/java.lang.String$CaseInsensitiveComparator.compare(String.java:1237) ~[na:na]
    at java.base/java.util.TreeMap.getEntryUsingComparator(TreeMap.java:374) ~[na:na]
    at java.base/java.util.TreeMap.getEntry(TreeMap.java:343) ~[na:na]
    at java.base/java.util.TreeMap.get(TreeMap.java:278) ~[na:na]
    at org.hibernate.dialect.function.SQLFunctionRegistry.findSQLFunction(SQLFunctionRegistry.java:45) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.ast.util.SessionFactoryHelper.findSQLFunction(SessionFactoryHelper.java:385) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.ast.tree.IdentNode.getDataType(IdentNode.java:367) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.ast.HqlSqlWalker.lookupProperty(HqlSqlWalker.java:669) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.propertyRef(HqlSqlBaseWalker.java:1159) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectExpr(HqlSqlBaseWalker.java:2320) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectExprList(HqlSqlBaseWalker.java:2261) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectClause(HqlSqlBaseWalker.java:1523) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:602) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:330) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:278) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:276) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:192) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.hql.internal.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:144) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:113) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:73) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.engine.query.spi.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:162) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.internal.AbstractSharedSessionContract.getQueryPlan(AbstractSharedSessionContract.java:613) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.internal.AbstractSharedSessionContract.createQuery(AbstractSharedSessionContract.java:725) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at org.hibernate.internal.AbstractSharedSessionContract.createQuery(AbstractSharedSessionContract.java:114) ~[hibernate-core-5.4.32.Final.jar:5.4.32.Final]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:564) ~[na:na]
    at org.springframework.orm.jpa.ExtendedEntityManagerCreator$ExtendedEntityManagerInvocationHandler.invoke(ExtendedEntityManagerCreator.java:362) ~[spring-orm-5.3.12.jar:5.3.12]
    at com.sun.proxy.$Proxy82.createQuery(Unknown Source) ~[na:na]
    at org.springframework.data.jpa.repository.query.SimpleJpaQuery.validateQuery(SimpleJpaQuery.java:90) ~[spring-data-jpa-2.5.6.jar:2.5.6]
    ... 58 common frames omitted